This disclosure provides a polymer composite including a polymer, nanocellulose, and a compatibilizer, wherein the nanocellulose comprises cellulose nanocrystals and/or cellulose nanofibrils, and wherein the compatibilizer comprises a maleated polymer. In some embodiments, the nanocellulose includes lignin-coated nanocellulose. The polymer may be selected from polyethylene, polypropylene, polystyrene, polylactide, or poly(ethylene terephthalate). The maleated polymer may be selected from maleated polyethylene, maleated polypropylene, maleated polystyrene, maleated polylactide, or maleated poly(ethylene terephthalate. Other variations provide a process for compatibilizing a polymer with nanocellulose, comprising: providing a polymer; providing nanocellulose comprising cellulose nanocrystals and/or cellulose nanofibrils; providing a maleated polymer; and combining the polymer, the nanocellulose, and the maleated polymer, wherein the maleated polymer functions as a compatibilizer between the polymer and the nanocellulose.